The present invention relates generally to video signal processing by the insertion of a separately generated blanking data signal including blanking pulse, horizontal sync and, in the case of color television video signals, color burst signal components, into an input video signal without the introduction of transient signal distortion. In particular, the invention is directed to such a video signal processing circuit and method employing a signal shaping means for reducing the slopes of the front and rear edges of the blanking pulse portion to enable the modified video signal produced after insertion of the blanking data signal to be transmitted through a low pass filter without transient overshoot and ringing distortion of the output video signal. The video signal processing circuit of the present invention is especially useful in a television frame synchronizer or a television timebase correction system.
A television signal processor employing a separately generated digital blanking data signal which is inserted into a digital input video signal for greater accuracy and less distortion of the horizontal sync and color burst components of such blanking data, is described in the article "Digital Processing In the DPS-1" by J. Lowry et al in Society of Motion Picture and Television Engineers Digital Video, Volume 2, pages 43-63 published March 1979. Previous television frame synchronizer circuits employing digital techniques for the insertion of separately generated digital blanking data signals have the disadvantage that they introduce transient distortion including overshoot and ringing in the blanking pulse portion of the output video signal. The video signal processing circuit of the present invention overcomes this problem.
The video signal processing circuit and method of the present invention provides an externally generated blanking data signal which is inserted into the input video signal to provide more accurate horizontal sync and color burst signal components by replacing these signal components in the input video signal. The blanking data signal may be a digitally generated blanking data signal which is inserted into a digital input video signal produced when the analog input video signal is transmitted through an analog to digital converter and a digital processor circuit with memory. The resulting modified video signal is then transmitted through a signal shaping circuit such as a voltage controlled amplifier which reduces the slope of the front and rear edge portions of the blanking pulse portion to provide a shaped video signal which is transmitted through a low pass filter without transient signal distortion such as ringing and overshoot of the output video signal as is produced by prior digital blanking data insertion circuits. Thus, the step transition times of the front and rear edges of the blanking pulse portion of the video signal are increased so that they are greater than the period of the upper limit frequency of the low pass filter, in order to prevent such transient signal distortion of the output video signal.
The control voltage terminal of the voltage controlled amplifier used to shape the video signal may be connected to the output of a slew limited amplifier, such as that shown in U.S. Pat. No. 4,163,948 issued Aug. 7, 1979 to M. L. Rieger et al, or the output of a low pass filter with a sine squared pulse response such as that described in the article "A New Sine-Squared Pulse and Bar Shaping Network" by A. Kastelein from IEEE Transactions of Broadcasting, pages 84-89 Volume BC-16, No. 4, December 1980. The input of the slew limited amplifier or the low pass filter with sine squared response is selectively connected by an electronic switch to different gain control voltages including a blanking gain voltage source which is applied when the blanking data signal portion is received at the input of the voltage control amplifier.